I will try to make as a detailed description as possible as I can not figure out which step needs to be corrected.

I would like to create a landscape view as seen by someone on a sleigh ride down the hill.

It starts with a winter forest extreme close up then trees start to move to the left ever so slightly, then speed up down hill and then decelerate and zoom out into a thin line on the horizon while slowing down but still moving to the left of the screen.

The way I decided to create this scene:

I created a nice multi layered file in Illustrator with all the trees overlapping each other. The file is to the maximum artboard width allowed by Illustrator.

I imported this multi layered Illustrator file into After Effects as a Comp with separate layers.

I assigned slight movement in the layers from right to left at various speeds. It gives a dimension to the view as you see one line of trees passing behind another at slightly different speed. This step does not seem to create any problems.

I kept on building the continuous line of trees extending to the right by duplicating imported Illustrator layers until I reached the limit of After Effects Comp canvas size. I believe it was at either 30000 or 60000 pxls. The reason why I need trees so large is because at the beginning of the animation we see them at an extreme close up with all the leaves and branches visible. Then the forest line starts to move to the left and speeds up.

Eventually it speeds up and zooms out at the same time enough to reach its end on the right. By this time the trees are at 50% of the screen size. So I import my tree layers again and match them perfectly at the seam and assign speed and scale so it continues to move past the viewer to the left and zooms out into perspective.

And now everything goes wrong. I perfectly match the points of original tree line and the additional tree line. I see they nicely touch each other in the beginning of the movement and at the end.

However unexpectedly in the middle they start to overlap each other and move slightly faster. For example: Left end of second forest line touches the right end of first forest line at the beginning at 10 sec and touches it the same at the end at 20 sec. However at 15 sec it speeds up and jumps ahead the right end of the first forest line. I tried to fix it by making a key frame at 15 sec and aligning them again. It resulted it two smaller jumps at 12.5sec and 17.5sec. If I keep on keying between the frames the left end of the second forest line continues to make little jumps in between. If I try to adjust the zoom out part it again messes everything up. I did set up my anchor point to the left side of layer.

I thought the problem is with Scale, so I made layer 3D and started to reduce a Z dimension but got the same result. The layers jump on each other and it does not seem as one continuous movement.POTENTIAL PROBLEM: I did assign Easy Ease, Ease In an Ease Out, so my acceleration and deceleration have nice and smooth motion. However I tried to match the speed and velocity in Key Frame options window (double click Alt on PC opens this dialog window) between the original and the additional forest lines.

I can not make this forest line long enough in a static window as I reached the width limitation of After Efffects canvas size. I need it very large as it starts at extreme close up and then zooms out to a and extreme horizon.

I can not make it a moving line and keep on patching it up as movement does not match and lines start to speed up between the key frames and overlap each other.

I though the problem was the combination of side ways right to left movement and zooming out motion (which I tried both ways: Scale in 2D layer and Z Dimension in 3D Layer) but I can not match even the linear speed of the forest lines from right to left. Perhaps there is a problem with how I am trying to match it.

I will even rebuild the entire project if someone can list steps how to make one continuous line of patched up elements in sync, so they seem as one continuous image line.

Hard to really grasp the situation without a few screen grabs, but some general thoughts:

If all your elements are vector art from Illustrator, scale shouldn't be a factor. Set your Illustrator layers to Continuously Rasterize, and you can move as close as you like to the layers with the camera and they will still be sharp. Thus, you can work at an overall smaller scale, and not go beyond AE's 30000 pixel limitation. Which also means you won't need to frame-match two environments.

It sounds to me like you are overcomplicating the job. Personally, I wouldn't combine camera movements and zooms - the potential that varied focal lengths will upset something is too great. I'd just do everything with camera movement, using the X axis for left-right movement, and the Z axis to move closer or further toward your subjects. After Effects cameras mimic the way real world cameras work, so it's best to approach the job like a real world situation. Build the world, then animate the camera moving within that world.

Remember that you can use Precomposed/Nested compositions to greatly streamline your work. Make a clump of 10 or so trees in one comp, then use that pre comp nested multiple times within your main comp. Don't forget to collapse transformations of the pre comps.

What Andrew said. This is simply an awful way to approach animation. Isolate the individual animation steps in separate, nested comps - one for the motion, the otehr for the zoom/ scale, a main comp for your camera work. Similarly, simply use effects like Motion Tile, Offset and Transform to create scrolling effects. You are hopelssly over-complicating things.